Protein disulphide isomerase

Abstract

What is it? Protein disulphide isomerase is an enzyme with two interrelated activities: as an oxidoreductase, it can catalyse the formation, reduction and isomerisation of disulphide bonds; and as a polypeptide binding protein, it can function as a molecular chaperone which assists the folding of polypeptides. Transient association of PDI with nascent polypeptides during their folding prevents non-productive interactions from occurring, thereby increasing the yield of correctly folded protein molecules. PDI also forms more permanent associations with specific proteins, becoming a subunit of prolyl-4-hydroxylase or microsomal triglyceride transfer protein, depending on its partners.When did it first come to prominence? The enzymic activity of the protein was first discovered in the early 1960s by Anfinsen's group. PDI was purified to homogeneity in the 1970s, a cDNA clone was isolated in the 1980s and the protein has since been extensively characterised. More recently, attention has again focused on PDI following the identification of an oxidase, Ero1, which catalyses the oxidation of PDI in the endoplasmic reticulum (ER).What are its distinguishing features? PDI has two active sites, each of which includes the motif WCGHCK; a similar motif occurs in the active site of thioredoxin. This conserved motif is characterised by a pair of vicinyl cysteine residues which shuttle between the disulphide and dithiol form. The reactions that this enzyme catalyses require the individual active sites to be maintained in either the oxidised disulphide form, for disulphide bond formation, or the reduced dithiol form, for isomerisation or reduction of disulphide bonds (see Figure 1Figure 1). The redox state of the active site is determined by its reduction potential and this in turn determines the reaction that PDI is able to catalyse. How the active sites are maintained in either their reduced or oxidised state, and how the ER maintains an oxidising environment, have been the subject of intense speculation over the past 40 years; but it is only recently, with the discovery of Ero1, that we are beginning to understand the mechanisms underlying these processes.Figure 1Reactions catalysed by PDI.Note that the redox state of PDI determines the reaction catalysed. Oxidation will generate reduced PDI, which itself can be reoxidised by Ero1. Reduction or isomerisation of disulphide bonds requires reduced PDI for catalysis to occur. At present a reductive pathway for PDI has not been established, but this role could be fulfilled by low molecular weight thiols such as glutathione.View Large Image | View Hi-Res Image | Download PowerPoint SlideDoes it have any relatives? Yes – PDI is a member of a large family of proteins that are resident within the ER and are thought to be involved in oxidative protein folding. In mammalian cells, this family includes ERp57, ERp72, ERp44, PDIR, P5, ERdj5 and PDIp; and in yeast, it includes Eps1, Eug1, Mpd1 and Mpd2. The bacterium Escherichia coli has functional periplasmic homologues, where there is a clear distinction between an oxidative pathway and a reductive pathway. These include DsbA (oxidase) and DsbC/G (reductase or isomerase). These proteins have little sequence homology with PDI, but like PDI they have domains which exhibit structural similarity to thioredoxin.Does it have any undeserved claims to fame? Yes – PDI has been wrongly ascribed a number of different roles, mainly as a result of its ability to bind to hydrophobic affinity probes and its abundant representation in cDNA libraries. These roles include phosphoinositide-specific phospholipase C and glycosylation site binding protein.What are the most pressing outstanding questions about this enzyme? What is the structure of PDI and how does this change during reduction and oxidation? What is the mechanism of oxidation of PDI by Ero1? Does PDI act as an isomerase or reductase in vivo and does glutathione play a role in this process? What is the ultimate electron acceptor following the reduction of Ero1 by PDI?